Torque sensor

ABSTRACT

The present invention relates to a magnetostrictive sensor for measuring a torque in a shaft ( 1 ). The sensor comprises at least one active magnetostrictive region ( 2 ) on the shaft ( 1 ), a surface pattern in the magnetostrictive region such that it obtains anisotropic properties, a first means ( 5 ) arranged to generate a magnetic field varying in time in the magnetostrictive region ( 2 ) and a second means ( 9 ) arranged to sense variation in the permeability in the magnetostrictive region ( 2 ). Said magnetostrictive region ( 2 ) comprises a first layer ( 3 ) of a magnetostrictive material which is provided on the surface of the shaft ( 1 ) and that said surface pattern is formed by a second layer ( 4 ) of a non-magnetostrictive material comprising a low resistivity.

BACKGROUND OF THE INVENTION AND PRIOR ART

[0001] The present invention relates to a magnetostrictive sensor formeasuring torque in a shaft, wherein the sensor comprises at least oneactive magnetostrictive region of the shaft, a surface pattern on themagnetostrictive region such that it obtains anisotropic properties, afirst means arranged to generate a magnetic field varying in time in themagnetostrictive region and a second means arranged to sense variationsin the permeability in the magnetostrictive region, and wherein saidmagnetostrictive region comprises a first layer of a magnetostrictivematerial provided on the surface of the shaft.

[0002] Such magnetostrictive sensors are known and exist in a number ofdifferent embodiments. Usually, magnetostrictive sensors according tothe above comprise first means comprising a first winding provided in ayoke and extending around the magnetostictive region of the shaft. Acurrent varying in time is arranged to be supplied to the first windingsuch that a magnetic field varying in time is generated in themagnetostrictive region. The second means comprises in general a secondwinding, which is provided in the same yoke as the first winding.Hereby, a voltage is induced in the second winding with a value inproportion to the magnetic flux density. Since the permeability of themagnetostrictive region is changed when it is subjected to a torque, themagnetic flux density is also influenced. The voltage induced in thesecond winding may thereby be used for determining the magnitude of thetorque in the shaft.

[0003] The differences between different known magnetostrictive sensorsare principally the design of the magnetostrictive region and the way toobtain anisotropic properties therein.

[0004] U.S. Pat. No. 5,646,356 shows a magnetostrictive sensor formeasuring of torque in a shaft. Thin strips of a low resistivitymaterial have been applied to the surface of the shaft having an angleof 45° to the extension of the shaft. The use of this sensor isrestricted to measure torque in shafts which consist of materials havinggood magnetostrictive properties. Since drive shafts only in exceptionalcases are manufactured of material having good magnetostrictiveproperties, the use of this sensor is restricted.

[0005] U.S. Pat. No. 5,491,369 shows a magnetostrictive sensor formeasuring a torque applied to a shaft. In order to provide such a sensoron a shaft, a plurality of grooves are formed on the circumferentialsurface of the shaft. Then, the shaft is subjected to a heat treatmentsuch that it receives an increased strength. Thereafter a binder layeris applied before an active magnetostrictive material is provided on thecircumferential surface of the shaft. Consequently, the method requiresboth mechanical treatment and heat treatment of the load-carrying shaft,which makes it less attractive for many applications.

[0006] JP 4-221 726 shows a magnetostrictive sensor for measuring torquein a shaft. The sensor comprises a magnetostrictive region having afirst layer of nickel which abuts the surface of the shaft and a secondlayer of permalloy, which is a ferro-magnetic material having a veryhigh permeability, provided on the first layer. After the application ofthe second layer on the first layer, fine strips of the second layer areremoved such that the magnetostrictive region obtains a surface pattern,which gives the magnetostrictive region anisotropic properties.

[0007] The second layer must be magnetostrictive for the function of thesensor and a considerable disadvantage is that the second layer alsomust have a high permeability.

[0008] CN 1030642 shows a magnetostrictive torque sensor having a firstlayer of copper provided on top of a circumferential surface of a shaftand thin strips of a magnetostrictive alloy provided thereon. Also inthis case, the applied strips must have a high permeability for thefunction of the sensor.

[0009] JP 10-176 966 shows a magnetostrictive torque sensor which has afirst layer of a magnetostrictive material which is provided on acircumferential surface of a shaft. The magnetostrictive sensor heredescribed is based on geometric anisotropy, which is provided since thefirst layer forms a geometric pattern on the surface of the shaft. Theobject of this invention is to improve the strength of the first layerregarding breakage and separation. Therefore, the first layer isprovided with a gradually decreasing thickness towards its end portions.Thereafter, a second layer of a non-magnetostrictive material isprovided such that it extends over the end portions of the first layerhaving a decreasing thickness. Thereby, the strength of the first layeris improved regarding breakage and separation. Consequently, thefunction of the second layer is only to provide a favourabledistribution of mechanical stress in the first layer in a mechanicalmanner. In the same manner as the sensors according to the above-citedJP4-221 726 and CN 1030642, the applied strips require a highpermeability for the function of the sensor.

SUMMARY OF THE INVENTION

[0010] The object with the present invention is to provide amagnetostrictive sensor for measuring a torque in a shaft, which sensoris simple to provide on a shaft, provides good measurement results andis possible to provide on existing shafts substantially independent ofthe manufacturing material of the shaft.

[0011] This object is achieved by the magnetostrictive sensor of theinitially mentioned kind which is characterised in that said surfacepattern is formed by a second layer of a non-magnetostrictive materialcomprising a low resistivity. Because of eddy currents induced in thefirst layer, the applied magnetic field decreases(is damped)exponentially with the distance from the surface of the first layer. Bychoosing a sufficiently thick layer, the properties of the sensor willbe dominated by the first layer material and not by the shaft material.By providing a first layer of a suitable thickness on thecircumferential surface of the shaft, the influence of the shaftmaterial on the measurement results of the sensor becomes more or lessnegligible. Thereby, the sensor may be provided on substantially allkinds of shafts and independent of shaft material. Such amagnetostrictive sensor also obtains a good function with a first layerof a material having a relatively low permeability.

[0012] According to a preferred embodiment of the present invention,said second layer is provided on the first layer. By providing a secondlayer of a non-magnetostrictive material on the first layer, the surfacepattern which provides the magnetostrictive region with anisotropicproperties is obtained. Advantageously, said first layer has acontinuous extension in said region. Such a first layer is simple toprovide at the same time as it forms a continuous and even underlyingsurface on which the second layer may easily be provided. Furthermore,such a continuous first layer minimises stress concentrations in themagnetostrictive region. According to an alternative embodiment, thefirst layer is provided with a non-continuous extension on the surfaceof the shaft and in that at least any portion where the first layer doesnot abut the surface of the shaft, the second layer is provided on thesurface of the shaft. By providing both the first layer and the secondlayer on the surface of the shaft in said surface pattern, themagnetostrictive region may obtain a substantially even surface layer.

[0013] According to another preferred embodiment said first and secondlayers are arranged to be applied by an application method whichessentially does not introduce heat. By avoiding high temperatures, thestrength and tolerances of the load-carrying shaft are not influenced. Asuitable application method which does not introduce heat is plating.There exists at least two applicable plating methods for plating metalson an existing element, namely bath plating and selective plating, butalso chemical (electro-less) plating is applicable. Advantageously, thesurface of the shaft is arranged to be subjected to a pre-treatmentbefore said layers are applied. By such a pre-treatment, which maycomprise blasting, shot peening, grinding, pickling, doping or chemicalstrike, the first layer obtains a sufficient bond to the surface of theshaft. Advantageously, said applied layers are arranged to be subjectedto an additional post-plating treatment. Such a treatment may bemechanical and/or thermal for improving the strength of the layers andthe properties of the sensor.

[0014] According to another preferred embodiment of the presentinvention, said first layer has a thickness which is larger than theskin depth on the magnetostrictive material. The skin depth of amaterial is a well known definition in the present technical field andis calculated according to the present formula

δ={square root}{square root over (2ρ/μω)}

[0015] where

[0016] δ=the skin depth,

[0017] ρ=the electric resistivity of the material,

[0018] μ=the magnetic permeability of the material and

[0019] ω=the angular frequency of the applied field.

[0020] In a first layer having a thickness of a skin depth, about ⅓ ofthe applied magnetic field penetrates through the first layer into theunderlying shaft material. In order to guarantee a good accuracy ofmeasurement, it is preferable to apply such a thick layer that only alesser amount of the applied magnetic field penetrates through the firstlayer into the shaft material. However, providing a first layerthickness which is greater than two skin depths usually contributes verylittle with respect to the accuracy of measurement since, at two skindepths, only a very small amount of the applied magnetic fieldpenetrates into the underlying shaft material. A greater layer thicknessimplies in general a higher manufacturing cost. Therefore, a first layerhaving a thickness between one and two skin depths is optimal for mostsensors. However, during circumstances favourable in other respects, arelatively good accuracy of measurement may be obtained also with athickness of the first layer as little as ¼ of the skin depth of themagnetostrictive material. Advantageously, said first layer comprises atleast one or more of of the materials nickel, iron or cobalt. Inparticular nickel has in a pure state or in alloys good magnetostrictiveproperties, in combination with that it may be applied withoutdifficulty on most kinds of shaft materials by, for example, plating.

[0021] According to another preferred embodiment of the presentinvention, said second layer comprises a material having a resistivitywhich is lower than the resistivity of the material of the first layer.With reference to the above mentioned U.S. Pat. No. 5,646,356, it isshown that the quotient between the resistivity of the material in thesecond layer and the resistivity of the material in the first layer aswell as the thickness of the second layer are of significance for howwell the applied field is aligned with respect to the surface pattern ofthe second layer. It is suitable that said second layer has a thicknesswhich is smaller than the skin depth in the first layer and larger thanthe skin depth multiplied by the quotient between the resistivity of thematerial in the second layer and the resistivity of the material in thefirst layer. Advantageously, a material in the second layer is chosenwhich has as low resistivity as possible. With a low resistivity, thesecond layer may be made very thin. Said second layer may comprisestrips arranged in parallel which form an angle of between ±20° and ±75°to a generatrix to the surface of the shaft. For the magnetostrictiveregion to obtain an optimal anisotropy, said angle should be ±45°. Atthese angles, the magnetic field coincides at the shaft surface with theprincipal mechanical stress directions when the shaft is loaded with atorque in either direction. Said second layer may be provided within atleast two zones, which comprise strips having different angles. Thestrips may, for example, have an angle of +45° to said generatrix in afirst zone while the strips have an angle of −45° to said generatrix ina second zone. Said strips may extend axially in a continuous waybetween said zones. The total length of the sensor then becomes minimal.It is also possible that the strips are interrupted between two zones.Advantageously, said second layer comprises one or more of materialscopper, aluminium or chrome. In particular copper has a very lowresistivity but aluminium and chrome also have a low resistivity. Copperand chrome may be applied without difficulty on top of a first layer of,for example, nickel, by plating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In the following a preferred embodiment of the present inventionis described as an example with reference to the attached drawings, inwhich:

[0023]FIG. 1 shows a magnetostrictive sensor for measuring a torque in ashaft according to the present invention and

[0024]FIG. 2 shows a cross-section of a shaft surface in amagnetostrictive region according to a first embodiment of the presentinvention.

[0025]FIG. 3 shows a cross-section of a shaft surface in amagnetostrictive region according to a second embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

[0026]FIG. 1 shows a rotatable shaft 1, which is arranged to transmit atorque in any kind of mechanical transmission. The shaft 1 may hereconsist of a substantially arbitrary material, which meets themechanical demands required on the shaft 1 as a transmission elementwith reference to hardness, strength and geometry. To be able to measurethe size and direction of a torque in the shaft 1, a magnetostrictivesensor has been provided in a suitable place along the extension of theshaft 1. In this case, the magnetostrictive sensor comprises amagnetostrictive region 2 provided on the shaft 1, which has anextension on the circumferential surface of the shaft. Themagnetostrictive region 2 comprises a first layer 3 of amagnetostrictive material, which has a substantially continuousextension and thickness in said region 2. Advantageously, the firstlayer 3 comprises nickel but it may also comprise iron, cobalt and othermaterials which have suitable magnetostrictive properties. A secondlayer 4 in form of a continuous strip is provided on the first layer 3.The strip comprises two end portions 4 a, b which each form an angle ofabout +45° to a generatrix to the surface of the shaft 1 and anintermediate portion 4c forming an angle of about −45° to a generatrixto the surface of the shaft 1. The second layer 4 comprises a materialwith non-magnetostrictive properties and it advantageously has a lowresistivity, low permeability and is easily applicable as a thin layeron the first layer. Copper is such a material but it may also bealuminium, chrome or any other material, which has the above-mentionedmaterial properties. The second layer 4 with said strips forms a surfacepattern on the first layer 3 such that the magnetostrictive region 2obtains anisotropic properties. Since the strip has an extension makingangles of ±45° to a generatrix to the surface of the shaft 1 an optimalanisotropy is obtained in connection with a torque load on the shaft 1.Such a torque loading may occur in two possible torque directions.

[0027] The second layer 4, in FIG. 1, consists of a pattern in form ofstrips having an angle of either +45° or −45° to the generatrix to theshaft. The second layer 4 forms here a pattern in the form of stripswhich extend across three zones with different angles. The number ofsuch zones may be one, two, three or more. The zones may either becontinuous such that the strips form a continuous unit or separated suchthat an interruption of the strips is formed between each zone. Theunderlying first layer 3 may also consist of only one continuous portionor several separated portions. If the first layer forms only onecontinuous portion or several separated portions, the second layer 4does not necessarily need to have a corresponding pattern. Consequently,an embodiment may consist of a continuous portion of the first layer 3and separated strips of the second layer 4. The most preferableembodiment in general, is that having a continuous portion of the firstlayer 3 and continuous strips of the second layer 4 since that makes thetotal length of the sensor the shortest possible. This is an advantagesince space is often very restricted in the environments where thesensor is used.

[0028] Furthermore, a load-carrying shaft may also be provided withseveral magnetostrictive regions, which enable torque measurement atseveral places along the shaft. This makes it possible to measure thetorque, for example, before and after a torque-transmitting element as,for example, a gear transmission or a pulley. In this way, thetransmitted torque in the torque-transmitting element may be calculatedas the torque difference between the two magnetostrictive regions.Torque measurement in several places along a shaft may also be ofinterest on other shafts, which have a varying torque along theextension of the shaft.

[0029] In order to generate a time varying magnetic field in saidmagnetostictive region 2, the sensor comprises a primary coil 5 which isarranged around the shaft 1. The primary coil 5 is installed in asubstantially cylindrically shaped yoke 6 of a soft magnetic material.The yoke 6 also encloses two poles 7, 8, which concentrate the flux inan air gap at the ends of the yoke 6. The primary coil 5 is connected toa signal generator, which is not shown in the figures. The signalgenerator is arranged to supply a current varying in time to the primarycoil 5 such that a magnetic field varying in time with a substantiallyuniform distribution may be applied to the magnetostrictive region 2. Asecondary coil 9 is arranged inside the primary coil 5 to measure themagnetic flux density. When the shaft 1 is subjected to torque, thepermeability of the magnetostrictive material is changed in the firstlayer and thereby also the magnetic flux. Depending on the amplitude ofthe magnetic flux, a corresponding induced voltage is produced in thesecondary coil 9. The induced voltage in the secondary coil 9 ismeasured and used for determining the size and direction of the torquein the shaft 1.

[0030]FIG. 2 shows a cross section view through a surface of the shaft 1in the magnetostrictive region 2. A first layer 3 of themagnetostrictive material has been provided on the circumferentialsurface of shaft 1 in a relatively thin layer with a continuousextension and thickness. Said first layer 3, which with advantagecomprises nickel, is advantageously applied by means of an applicationmethod which does not introduce heat during the application process,which for example may be plating. An application method, which does notintroduce heat, is favourable since the introduction of heat during theapplication process may result in that the strength and themanufacturing tolerances of the shaft are changed in an unfavourablemanner. In order to ensure that the first layer 3 obtains a necessarybond to the surface of the shaft, the circumferential surface of theshaft may possibly be pre-treated. Such a pre-treatment may compriseblasting, shot peening, grinding, pickling, doping or chemical strike.After the first layer 3 is applied to the surface of the shaft, thesecond layer 4 is applied on the first layer 3. Consequently, such anapplication is with advantage performed by plating. In order to obtainthe necessary surface pattern for the function of the sensor, theregions which are not intended to be applied with the second layer 4may, for example, be masked. Conceivable or preferable bath forNi-plating is a Watt's Bath, a sulphamate bath or a high chloride bath.The plating may be followed by a stabilising heat treatment at between150° and 300° C. In certain applications a higher temperature may alsobe considered. Besides the stabilising heat treatment (baking) of thefirst layer 3, a mechanical post-plating treatment may also bepreferable in order to stabilise or improve the properties of thesensor, improve the strength, change inner stresses or improve fatiguestrength of the first layer 3. Examples of such additional post-platingtreatment are blasting, shot peening, roll polishing or any othercorresponding method. The mechanical treatment may be performed eitherseparately or before or after the heat treatment (baking) or in anysequence of treatments consisting of mechanical and thermal treatment.

[0031] In order to obtain a sensor that functions well at a relativelylow cost, the thickness of the first 3 and second 4 layers are of greatimportance. Consequently, the part of the applied magnetic field, whichpenetrates through the first layer 3 influences not only themagnetostrictive material but also the shaft material. Since the shaftmaterial usually has considerably inferior magnetostrictive propertiesthan the first layer 3, a thin first layer 3 decreases the accuracy ofthe sensor. Therefore, a thick first layer 3 is favourable from thepoint of view of measurement. However, the cost increases with thethickness of the applied layer. A well-known quantitative parameter inthis technical field is skin depth. A skin depth concerns the depth in amaterial, at which a remainder of about ⅓ (more exactly a factor 1/e) ofthe applied magnetic field amplitude penetrates further down in thematerial. A thickness of the first layer 3 between one and two skindepths, usually leads to a very good accuracy of measurement combinedwith a relatively low cost for application of the first layer 3.However, a relatively good accuracy of measurement may also be performedwith a thickness of the first layer 3 as little as ¼ skin depth. Withsuch a thin first layer 3 as ¼ of the skin depth of the magnetostrictivematerial, (only) 31% of the magnetic flux is in the first layer 3. Witha sensitivity (change of permeability caused by mechanical load) in thefirst layer 3 which, for example, may be 5 times as large as in theunderlying shaft material, the first layer 3 will nevertheless dominatethe properties of the sensor since 69% of the sensitivity depends on thefirst layer 3. In that connection such layers 3 as thin as ¼ of the skindepth may be commercially interesting.

[0032] In view of cost, it is also preferable to have a thin secondlayer 4. In order to obtain a sensitive sensor having a thin secondlayer 4, which forms said surface pattern on the first layer 3, amaterial ought to be used in the second layer 4 which has a very lowresistivity. The second layer 4 ought to be a material, which has atleast a lower resistivity than the resistivity of the material in thefirst layer 3. Advantageously, the second layer 4 has a thickness whichis less than the skin depth in the first layer 3 and greater than theskin depth multiplied by the quotient between the resistivity of thematerial in the second layer 4 and the resistivity of the material inthe first layer 3.

[0033] In order to improve the properties of the magnetostrictive layerparticularly at high loads, it is important how the edge 4 of this layeris designed at the end towards the underlying shaft 1. A suitable designis such that the mechanical stress concentrations are minimised. Forsmall loads, the edges of the layer 4 may be entirely straight, which isshown in FIG. 2, but for higher loads they ought to be designed with achamfer, a radial transition or any other smooth transition from thelayer to the shaft 1. The most optimal design is determined by theperformance of the shaft 1, the mechanical stress levels and availablespace.

[0034]FIG. 3 shows a cross-section view of an alternative embodiment ofa magnetostrictive region 2. The first layer 3 here has been provided onthe surface of the shaft 1 with a non-continuous extension. The secondlayer 4 has been provided on the surface of the shaft 1 in the portionswhere the first layer 3 does not abut the surface of the shaft 1.Thereby, said first 3 and second 4 layers form a geometricallycontinuous material layer on the surface of the shaft 1. The first 3 andsecond 4 layers in the embodiment shown have a thickness of the samesize and the magnetostrictive region 2 obtains thereby an even surface.

[0035] A torque sensor according to the present invention may be used asa separate component in all such applications where one for differentreasons is interested in measuring torque. The torque measurement may,for example, be used for supervision or control or any combinationthereof. It may also consist a part of an overload protection.

[0036] The torque sensor may be permanently mounted in the machine ormounted on or during the occasion when the torque is to be measured. Itmay also be integrated directly in a mechanical equipment in such amanner that it consists a part of a load carrying component, whichalready exists in the machine. This component may be designed accordingto its original design or may be modified to facilitate the torquemeasurement. It may also consist of a component which has been suppliedin order to enable torque measurement and which is adapted to fit in themechanical equipment.

[0037] The sensor may be integrated in industrial machines such asturning or milling machines, drills, nut runners etc. It may also beused for calibration of other tools or other instruments. Furthermore,it may be used in mixers, extruders, agitators, actuators orviscosimeters. It may also be used in other kinds of manufacturingmachines having rotating or non-rotating shafts. In the above-mentionedapplications, the torque signal may be used for, for example,supervision and control.

[0038] It may also be used in vehicles such as, for example, cars,trucks or construction equipment. It may be used to measure torque inthe engine, on the crank shaft, on the input shaft to the gearbox, inthe gearbox, on the output shaft from the gearbox, on the propellershaft, in the differential gear, on the driving shaft, in the wheel orany any other part of the power train. It may be used to monitor thetorque or to use the torque to control engine, gearbox, clutch or in anyother way influence the power train. It may also be used to measuresteering torque in a steering wheel, steering rod or in a steering gearfor supervision of the steering torque or to control a power steering.

[0039] Furthermore, the sensor may also be used in, for example,bicycles for measuring the performance of the cyclist or to control anelectrical or in another manner power assisted bicycle. The torquesensor may also be used in aeroplanes, jet engines, propeller engines orhelicopters. Also other applications using torque in rotating shafts asenergy transmitting or power transmitting elements may be used with thetorque sensor. The sensor may also be used for torque measurement innon-rotating shafts.

[0040] The present invention is not in any way restricted to theembodiments shown on the drawings but may be varied freely within thescope of the claims. For example, the first layer may have a continuousextension in the magnetostrictive region with a variable layerthickness. Hereby, the first layer may have a larger thickness inportions between applied second layers. Thereby, the magnetostrictiveregion may obtain an essentially even surface. Furthermore, such amagnetostrictive region does not need to extend entirely around thecircumferential surface of the shaft but may extend only around a partof the shaft. In addition, said primary and secondary windings mayconsist of the same winding. Except that, the material of said firstlayer does not necessarily need to be applied as a finished layermaterial but the possibility exists to use different methods toinfluence or to change the material of the shaft to form said firstlayer material. In addition, other methods than plating may be used forapplying said first and second layers. Such methods may be physicalvapour deposition (PVD), chemical vapour deposition (CVD), plasmaspraying, sputtering and laser coating. Preparation of the surface ofthe shaft and/or additional treatment of said layers may be applicablefor these methods as well. Finally, the shaft does not necessarily needto be rotatable but it may also be static.

1. A magnetostrictive sensor for measuring a torque in a shaft (1),wherein the sensor comprises at least one active magnetostrictive region(2) on the shaft (1), a surface pattern on the magnetostrictive region(2) such that it obtains anisotropic properties, a first means (5)arranged to generate a time varying magnetic field in themagnetostrictive region (2) and a second means (9) arranged to sensevariations in the permeability in the magnetostrictive region (2), andwherein said magnetostrictive region (2) comprises a first layer (3) ofa magnetostrictive material provided on the surface of the shaft (1),characterised in that said surface pattern is formed by a second layer(4) of a non-magnetostrictive material having a resistivity, which islower than the resistivity of the material of the first layer (3).
 2. Amagnetostrictive sensor according to claim 1, characterised in that thesecond layer (4) is provided on the first layer (3).
 3. Amagnetostrictive sensor according to claim 1 or 2, characterised in thatsaid first layer (3) has a continuous extension in said region (2).
 4. Amagnetostrictive sensor according to claim 1, characterised in that thefirst layer (3) is provided with a non-continuous extension over thesurface of the shaft (1) and in that at least any portion where thefirst layer (3) does not abut the surface of the shaft (1), the secondlayer (4) is provided on the surface of the shaft (1).
 5. Amagnetostrictive sensor according to any one of the preceding claims,characterised in that said first (3) and second layers (4) are arrangedto be applied by an application method, which essentially does notintroduce heat.
 6. A magnetostrictive sensor according to claim 5,characterised in that said application method is plating.
 7. Amagnetostrictive sensor according to claim 5 or 6, characterised in thatthe surface of the shaft (1) is arranged to be subjected to apre-treatment before said layers are applied.
 8. A magnetostrictivesensor according to any one of the claims 5-7, characterised in thatsaid applied layers are arranged to be subjected to a post-applicationtreatment.
 9. A magnetostrictive sensor according to any one of thepreceding claims, characterised in that said first layer (3) has athickness which is greater than ¼ of the skin depth of themagnetostrictive material.
 10. A magnetostrictive sensor according toclaim 9, characterised in that said first layer (3) has a thicknesswhich is greater than the skin depth of the magnetostrictive material.11. A magnetostrictive sensor according to any one of the precedingclaims, characterised in that the first layer (3) comprises one or moreof the materials nickel, iron or cobalt.
 12. A magnetostrictive sensoraccording to any one of the preceding claims, characterised in that saidsecond layer (4) comprises a material having a permeability which islower than the permeability of the material of the first layer (3). 13.A magnetostrictive sensor according to any one of the preceding claims,characterised in that said second layer (4) has a thickness which isless than the skin depth in the first layer and greater than the skindepth multiplied by the quotient between the resistivity of the materialin the second layer (4) and the resistivity of the material in the firstlayer (3).
 14. A magnetostrictive sensor according to any one of thepreceding claims, characterised in that said second layer (4) comprisesstrips arranged in parallel, which form an angle of between ±20° and 75°to a generatrix to the surface of the shaft.
 15. A magnetostrictivesensor according to claim 14, characterised in that said strips areprovided in at least two zones, which comprises strips with differentangles.
 16. A magnetostrictive sensor according to claim 15,characterised in that said strips extend in a continuous way betweensaid zones.
 17. A magnetostrictive sensor according to claim 15,characterised in that said strips extend with an interruption betweensaid zones.
 18. A magnetostrictive sensor according to any one of thepreceding claims, characterised in that said second layer (4) comprisesone or more of the materials copper, aluminium or chrome.
 19. Use of amagnetostrictive sensor according to any one of the preceding claims formeasuring a torque in a shaft.